Reproducing materials advantageously utilizing the photosensitivity of diazo compound are widely in use due to their low cost, and are known by the following three major types.
The first type, known as the wet developing type, is formed by a photosensitive layer comprising a diazo compound and a coupling component as the main constituents on the support. After this type of reproducing materials is laid over the original to be reproduced and exposed to light, it is developed with an alkaline solution.
The second type, known as the dry developing type, is developed with ammonia gas, rather than the alkaline solution used by the wet developing type.
The third type known as the heat-developing type has several variations. These variations include the type which contains an ammonia gas generating agent like urea, which upon heating, generates ammonia gas in the photosensitive layer; the type which contains an alkaline salt of compounds such as trichloroacetic acid, which lose their acidic characteristics upon heating, in the photosensitive layer, or the type in which thermal fusion activates a diazo compound and a coupling component, and which uses higher fatty amide as coloring aid.
The wet type of recording materials have a number of disadvantages, including that it is impossible to make additional entries soon after copying because the recording material is still wet, and that copied images cannot tolerate long storage. In addition, there are difficulties in maintenance and control, such as replenishing and disposing of the developing agent due to its liquid form, and the large size of the equipment.
The dry type materials also result in problems with replenishing the liquid developer, similar to the wet type. In addition, this type has the disadvantage of requiring an ammonia gas absorbing device in order to prevent leakage of the generated gas to the outside, this in turn requires large equipment and creates a strong odor of ammonia immediately after reproducing.
On the other hand, the heat-developing type, in contrast to the wet type and the dry type, provides an advantage in maintenance by eliminating the use of a developing liquid, although it has the disadvantages of possible underdevelopment and variable color tones due to the required high developing temperature of 150.degree. C. to 220.degree. C. with a temperature control tolerance of .+-.10.degree. C., which requires costly equipment to ensure the reproduction of high-quality images. Furthermore, in order to withstand such a high developing temperature, the diazo compound used in this type must have high heat resistance, which is often a disadvantage in obtaining the high density of recorded images.
Therefore, various attempts have been made to achieve low temperature development between 90.degree. C. and 130.degree. C., but these attempts resulting in shorter shelf lives in the recording materials. Thus, the heat-developing type of reproducing materials have not yet become a widely used type of diazo-type reproducing system, although it is well known that heat-developing type has more advantageous maintenance properties when compared to the wet type and the dry type.
On the other hand, user's needs are continuously being diversified. For example, the user may need to not only obtain colored images on a white base, but also need to choose hues for the base texture and colored images to match depending on the uses. This is because it is necessary to draw the user's attention to the reproduced items when the recording material is used for drawings or posters, however, the existing recording materials are not capable of meeting such needs.
In order to obtain the desired color density through heating a layer comprising a diazo compound, a coupling component and a coloring aid provided on the support, it is necessary to form the dyes by instant solving, dispersing and reacting the respective components through heating.
In this case, however, if a recording material was to be designed such that it is possible to develop colors at low temperatures and produce high density images, the coloring reaction would gradually proceed at room temperature during storage before the material is used. This would cause the base texture to be spoiled with colors, instead of keeping the base texture clean white.
In the present invention, these above-mentioned problems, which look incompatible at a glance, have been overcome by enclosing the diazo compound component in microcapsules on the reproducing material in a photosensitive layer comprising a diazo compound, a coupling component and a coloring aid provided on the support, which can be developed by heating (refer to U.S. Pat. No. 4,529,681).
However, even in this case, if images of considerably high density are desired, the period of time in which the recording material can be maintained before it is used for this purpose is insufficient.